The present invention relates generally to transmitter circuitry, and, more particularly, to a Differential Quadrature Phase Shift Keying (DQPSK) modulation circuit for generating a DQPSK-modulated signal.
A communication system which transmits information between two locations includes, at a minimum, a transmitter and a receiver interconnected by a transmission channel. An information signal (which contains information) is transmitted by the transmitter upon the transmission channel to the receiver which receives the transmitted, information signal.
A radio communication system comprises one type of communication system wherein the transmission channel is formed of a radio-frequency channel. The radio-frequency channel is defined by a range of frequencies of the electromagnetic frequency spectrum. To transmit an information signal upon the radio-frequency channel, the information signal must be converted into a form suitable for transmission thereof upon the radio-frequency channel.
Conversion of the information signal into the form suitable for transmission thereof upon the radio-frequency channel is accomplished by a process referred to as modulation wherein the information signal is impressed upon a radio-frequency electromagnetic wave. The radio-frequency electromagnetic wave is of a frequency of a value within the range of values of frequencies which defines the radio-frequency channel. The radio-frequency electromagnetic wave upon which the information signal is impressed is commonly referred to as a "carrier signal", and the radio-frequency electromagnetic wave, once modulated by the information signal, is referred to as a modulated, information signal, or, more simply, a modulated signal.
The information content of the resultant modulated signal occupies a range of frequencies, sometimes referred to as the modulation spectrum, centered at, or close to, the frequency of the carrier signal. Because the modulated signal may be transmitted through free space upon the radio-frequency channel to transmit thereby the information signal between the transmitter and the receiver of the communication system, the transmitter and the receiver need not be positioned in close proximity with one another. As a result, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
Various modulation techniques have been developed to modulate the information signal upon the carrier signal to form the modulated signal, thereby to permit the transmission of the information signal between the transmitter and the receiver of the radio communication system. Such modulation techniques include, for example, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency-shift keying modulation (FSK), phase-shift keying modulation (PSK), and continuous phase modulation (CPM). One type of continuous phase modulation is quadrature amplitude modulation (QAM). To be discussed more fully hereinbelow is a particular QAM modulation technique, namely, a differential quadrature phase shift keying (DQPSK) modulation technique.
The receiver of the radio communication system which receives the modulated signal contains circuitry to detect, or to recreate otherwise, the information signal modulated upon the carrier signal. Typically, the circuitry of the receiver includes circuitry (sometimes consisting of several stages) to convert downward in frequency the modulated signal received by the receiver in addition to the circuitry required to detect, or to recreate otherwise, the information signal. The process of detecting or recreating the information signal from the modulated signal is referred to as demodulation, and such circuitry for performing the demodulation is referred to as demodulation circuitry. The circuitry of the receiver is constructed to detect, and to demodulate modulated signals which have been modulated by one of the modulation techniques above-mentioned.
In many instances, a plurality of modulated signals may be simultaneously transmitted as long as the simultaneously-transmitted, modulated signals are formed of carrier signals of dissimilar frequencies, and the resultant, modulated signals do not overlap in frequency. More particularly, the frequencies of the carrier signals of the simultaneously-transmitted, modulated signals must be separated, in frequency, to prevent the information content of the resultant, modulated signals (i.e., the modulation spectrum of the modulated signals) from overlapping with simultaneously-transmitted, modulated signals modulated upon carrier signals of other frequencies.
The broad range of frequencies of which the carrier signal may be comprised, and upon which the information signal may be modulated, is referred to as the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands, and have regulated transmission of modulated signals upon various ones of the frequency bands. The frequency bands have been further divided into channels, and such channels form the radio-frequency channels of a radio communication system. Regulation of the transmission of modulated signals within the various ones of the frequency bands, and, more particularly, upon the channels into which the frequency bands have been divided, minimizes interference between simultaneously-transmitted, modulated signals.
For instance, portions of a one hundred MHz frequency band extending between eight hundred MHz and nine hundred MHz are allocated in the United States for radiotelephone communication. Such radiotelephone communication includes radiotelephone communication in a cellular, communication system. Conventionally, a radiotelephone contains circuitry to permit simultaneous generation and reception of modulated signals, to permit thereby two-way communication between the radiotelephone and a remotely-located transceiver.
A cellular, communication system is formed by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated signals transmitted by one, or many, radiotelephones, and to transmit modulated signals to the one, or many, radiotelephones.
The positioning of each of the base stations forming the cellular, communication system is carefully selected to ensure that at least one base station is positioned to receive a modulated signal transmitted by a radiotelephone positioned at any location throughout the geographical area. That is to say, at least one base station must be within the transmission range of a radiotelephone positioned at any such location throughout the geographical area. (Because the maximum signal strength, and hence, maximum transmission range, of a signal transmitted by a base station is typically greater than the maximum signal strength, and corresponding maximum transmission range, of a signal generated by a radiotelephone, the maximum transmission range of a signal generated by a radiotelephone is the primary factor which must be considered when positioning the base stations of the cellular, communication system.)
Because of the spaced-apart nature of the positioning of the base stations, portions of the geographical area throughout which the base stations are located are associated with individual ones of the base stations. Portions of the geographical area proximate to each of the spaced-apart base stations define "cells" wherein a plurality of cells (each associated with a base station) together form the geographical area encompassed by the cellular, communication system. A radiotelephone positioned within the boundaries of any of the cells of the cellular, communication system may transmit, and receive, modulated signals to, and from, at least one base station.
Increased usage of cellular, communication systems has resulted, in many instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular, radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communications. By more efficiently utilizing the frequency band allocated for radiotelephone communication, the transmission capacity of an existing cellular, communication system may be increased.
The transmission capacity of the cellular, communication system may be increased by minimizing the modulation spectrum of the modulated signal transmitted by a transmitter to permit thereby a greater number of modulated signals to be transmitted simultaneously. Additionally, by minimizing the amount of time required to transmit a modulated signal, a greater number of modulated signals may be sequentially transmitted.
By converting an information signal into discrete form prior to modulation thereof, and then modulating the discrete, information signal, the resultant, modulated signal is typically of a smaller modulation spectrum than a corresponding modulated signal comprised of an information signal that has not been converted into discrete form. Additionally, when the information signal is converted into discrete form prior to modulation thereof, the resultant, modulated signal may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
Converting the information into discrete form is typically effectuated by an encoding technique, and apparatus which effectuates such conversion is typically referred to as an encoder. An encoded signal generated as a result of an encoding technique may, for example, be in the form of a discrete binary data stream. The elements (i.e., bits) of the discrete binary data stream represent various characteristics of the information signal. The binary data stream comprising the encoded signal may be appropriately filtered, and modulated by a modulation technique, as noted hereinabove, to form a modulated signal of a frequency appropriate for transmission upon a desired transmission channel.
When an information signal is comprised of a voice signal (i.e., a speech waveform), an encoding technique not only converts the speech waveform into discrete form, but, additionally, in so doing, removes some of the significant redundancy of the speech waveform. Because the binary data stream into which the speech waveform is converted is representative of a speech waveform with some of the redundancy removed, the bandwidth of a resultant, modulated signal formed therefrom is less than the bandwidth of a corresponding, modulated signal formed of a non-encoded speech waveform. The smaller bandwidth of the resultant, modulated signal permits either more transmission channels to be defined over a frequency band of the same bandwidth; intermittent use of the transmission channel is additionally permitted rather than continuous use which is required for transmission of a modulated signal comprised of an information signal in which a non-encoded speech waveform comprises the information signal. More than one information signal may, hence, be transmitted upon a transmission channel; the transmission capacity of the existing frequency band allocated for radiotelephone communications may be increased by a multiple of two or more.
Encoding of an information signal into a discrete binary data stream is also advantageous as noise introduced upon the modulated signal during transmission upon the transmission channel may be more easily detected and removed when the information signal is comprised a discrete binary data stream than when the information signal is comprised of a conventional, analog signal.
Still further efficiency may be achieved by advantageous selection of the modulation technique utilized to modulate the information signal to form the modulated signal thereby. Traditionally, frequency modulation is the modulation technique utilized to form the modulated signal transmitted by or to a radiotelephone of a cellular, communication system. A modulated signal generated as a result of a frequency modulation technique passes information only by changing the phase of the carrier signal. Conversely, when the modulated signal is generated as a result of a continuous phase modulation technique, information is passed by changing both the amplitude and by changing the phase of the carrier signal.
Therefore, a radio communication system capable of generating and receiving modulated signals formed by a continuous phase modulation technique may more efficiently utilize the frequency band allocated therefor.
As mentioned hereinabove, one type of continuous phase modulation includes quadrature amplitude modulation (QAM). A binary data stream comprising the information signal may be advantageously modulated to form a baseband, QAM-modulated signal according to this modulation technique. The binary data stream is separated into bit pairs. The data stream is then passed through a pair of electric wave filters, and applied to a multiplier pair having second inputs comprised of sine and cosine components of a carrier or carrier intermediate frequency signal. One particular QAM modulation technique is a .pi./4-shift DQPSK (for differential quadrature phase shift keying) modulation technique. In a .pi./4-shift DQPSK modulation technique, an input data stream is encoded so that the composite modulated carrier phase shifts in increments of plus or minus of .pi./4 radians or plus or minus 3.pi./4 radians according to values of the individual bit pairs.
Once the baseband, QAM-modulated signal is formed, the modulated signal must be converted in frequency to a radio frequency. In conventional practice, for instance, transmitter apparatus capable of generating a QAM signal, includes circuitry which separates a carrier signal into sine and cosine components, mixes the sine and the cosine components, respectively, with separate portions of the baseband signal, and then sums output signals generated by the respective mixers. The resultant, modulated signal, if at carrier frequency, may be applied to an amplifier to amplify the signal to a desired power level prior to transmission thereof. If the resultant, modulated signal is at a carrier intermediate frequency, a LO oscillating signal is further mixed with the resultant, modulated signal to shift the resultant, modulated signal in frequency to a proper carrier frequency, and then amplified.
Conventionally, circuitry forming a finite impulse response (FIR) filter is utilized to form a filtered, DQPSK-modulated signal. Such circuitry, however, requires repeated multiplicative functions to be performed to form the filtered, DQPSK-modulated signal. Such circuitry, therefore, is complex and costly, and/or requires significant computational time periods to form the filtered, DQPSK-modulated signal.
What is needed therefore, is simplified means for generating a filtered, DQPSK-modulated signal.